High-rise buildings, indoor sports facilities, etc. are steel structure facilities in which giant space is required to be secured. As structural members for the same, high strength thick steel materials or giant H-shapes are being utilized. If steel plates or steel shapes increase in thickness, in particular, securing the amount of reduction at the center of the plate thickness becomes difficult and variations in material quality become a problem. Further, if securing hardenability by raising the carbon equivalent (Ceq), the weldability ends up falling.
To deal with this problem, methods of improving the weldability and toughness of high strength thick steel material are, for example, proposed in Japanese Patent Publication (A) No. 9-310117, Japanese Patent Publication (A) No. 2000-199011, Japanese Patent Publication (A) No. 2002-173734, etc.
The method proposed in Japanese Patent Publication (A) No. 9-310117 and Japanese Patent Publication (A) No. 2000-199011 reduces the amount of C, lowers the weld cracking susceptibility parameter (Pcm), and makes the metal structure a bainite single-phase structure or granular bainitic ferrite to reduce variations in material quality.
Further, the thick steel material proposed in Japanese Patent Publication (A) No. 2002-173734 is made of ingredients reducing the Ceq and Pcm and utilizes solid solution Nb to obtain a strength and toughness in accordance with the application.
Furthermore, an giant H-shape comprised of not just steel plate, but an extremely low carbon bainite structure into which quasi polygonal ferrite is dispersed is for example proposed in Japanese Patent Publication (A) No. 11-193440.
The methods proposed in these patent citations omit heat treatment and utilize controlled rolling to obtain giant H-shapes excellent in strength and toughness.